Project Z06: The Aftermath of Installing Longtube Headers

So you installed longtube headers and now you don’t know what to do with those rear oxygen sensors. Here’s the fix. It took us awhile to track this information down, so hopefully if someone does a search for "turn rear O2 sensors off LS HP Tuners", they'll find this article.

We recently installed a set of Texas Speed and Performance 1 3/4” longtube headers and catless off-road X-pipe on our C5 Z06 project car. We’ll deal with that actual install in a future update, but for now, let’s deal with the aftermath, which is just as important.

See, most longtube headers are only going to have bungs for one set of oxygen sensors, but your stock Corvette exhaust will have oxygen sensors both in front of and behind the catalytic converters. The front set are the ones you’re going to reuse. Most header suppliers will actually send you harness extensions so you can easily move those front sensors back to the bungs on the longtubes. Those front sensors are the ones that do the heavy lifting in determining air/fuel ratio and precisely delivering their info to the computer so it can make proper fuel and spark decisions. The rear set is mostly there to make sure the catalytic converters are working properly. Now that we’ve ditched the cats—or in the case that you are using high-flow cats—we need to make some adjustments to make sure everything tunes right.

So what needs to actually happen is for those rear oxygen sensors—which will not be hooked to anything and just left as empty wire sockets—to be “switched off” digitally, so the computer is not looking for signals that the sensors aren’t sending, or misinterpreting those non-signals as incorrect signals. We used HP Tuners for this operation, but the process will be similar in whatever tuning suite you use.

First you’ll need to disable the diagnostic codes referencing the rear sensors. In HP Tuners, under Engine Diag > DTCs, you’re going to switch off (uncheck SES Enable) the following codes:

You’ll also need to set the Error Mode to “No Error Reported” for all of these codes.

Finally, you’ll want to switch off 'COT', or Catalyst Over-Temp protection. This is a routine whereby if the rear oxygen sensors detect a high catalyst temperature, the computer dumps more fuel into the cylinders to cool the cats down. Without those sensors in place, the computer can default to this mode out of caution, dumping way more fuel into your engine at full throttle than you need for optimum power.

To access this function, go to Engine > Fuel > Temperature Control > Catalyst Protection and set 'COT' to “Disabled”.

So now your headers should be happily on the car and the car should be pleased to receive them. You’ll still need a tune to get those last few hidden hp, but this will at least get you running properly and out on the road.

Note that with these modifications and the defeat of many of the emissions control systems, we’ve crossed the line to where this is now a dedicated track car. While we’re lucky in Florida—we’d have no problem keeping this thing registered and street legal with our local laws—most folks have far more stringent emissions laws to deal with. Regardless, we like to keep our un-catted cars on the track as exclusively as possible, just to be good earthlings.

Comments

Jordan Rimpela Digital Editor
7/30/19 2:57 p.m.

It sounds great!

meldog21 New Reader
7/30/19 5:24 p.m.

Nice job switching to the headers and X-pipe with a good explanation. Now all you have to do is realign the mufflers so it doesn't look all cattywompus.

It might seem crazy, but I've found the only way to make the exhaust tips match requires 3 people. One guy to eyeball from about 15' behind the car, one guy to hold the mufflers in the right spot accounting for "spring back", and one guy to tighten the clamps. Or, you can just throw them up there and not worry about how the tips look.

JG Pasterjak Production/Art Director
7/30/19 7:53 p.m.

meldog21 said:
Nice job switching to the headers and X-pipe with a good explanation. Now all you have to do is realign the mufflers so it doesn't look all cattywompus.

It might seem crazy, but I've found the only way to make the exhaust tips match requires 3 people. One guy to eyeball from about 15' behind the car, one guy to hold the mufflers in the right spot accounting for "spring back", and one guy to tighten the clamps. Or, you can just throw them up there and not worry about how the tips look.

Oh yeah they're completely wonked in that video. They're about 60% better after tweak #1. i figure 2-3 more tries and they'll be near perfect :)

alfadriver MegaDork
7/31/19 6:33 a.m.

JG Pasterjak said:
Finally, you’ll want to switch off 'COT', or Catalyst Over Temp protection. This is a routine whereby if the rear oxygen sensors detect a high catalyst temperature, the computer dumps more fuel into the cylinders to cool the cats down. Without those sensors in place, the computer can default to this mode out of caution, dumping way more fuel into your engine at full throttle than you need for optimum power.

JG- who told you that?

The sensors don't measure temps. They generate a voltage based on the ratio of oxidants to reductants in the exhaust stream. Unless GM is using some trick sensors, they don't measure exhaust temp- that's generally modeled, which is actually a pretty strong model. The "dumping" of the fuel due to the removal of the sensor is a totally different thing that it thinks is being observed. Especially if you are talking about the rear sensors- which is actually a pretty darned key part in the emissions control- it's not there just to monitor the catalyst.

I've read some of the HP Tuners threads- there's some good direction there, but there's just as much that is very wrong.

While taking out the cat over temp protection is the correct thing to do for a motorsports calibration, the logic of why to do it is not right.... Just not related to the O2 sensors at all. (Well, except that they, too, have temperature protections put onto them, just like the catalyst and the exhaust manifold.)

StuntmanMike New Reader
7/31/19 7:42 a.m.

Sounds great! Did yall sneak a cam in there at some point and I missed it?

Jerry From LA SuperDork
8/2/19 2:24 p.m.

alfadriver said:
JG Pasterjak said:
Finally, you’ll want to switch off 'COT', or Catalyst Over Temp protection. This is a routine whereby if the rear oxygen sensors detect a high catalyst temperature, the computer dumps more fuel into the cylinders to cool the cats down. Without those sensors in place, the computer can default to this mode out of caution, dumping way more fuel into your engine at full throttle than you need for optimum power.

JG- who told you that?

The sensors don't measure temps. They generate a voltage based on the ratio of oxidants to reductants in the exhaust stream. Unless GM is using some trick sensors, they don't measure exhaust temp- that's generally modeled, which is actually a pretty strong model. The "dumping" of the fuel due to the removal of the sensor is a totally different thing that it thinks is being observed. Especially if you are talking about the rear sensors- which is actually a pretty darned key part in the emissions control- it's not there just to monitor the catalyst.

I've read some of the HP Tuners threads- there's some good direction there, but there's just as much that is very wrong.

While taking out the cat over temp protection is the correct thing to do for a motorsports calibration, the logic of why to do it is not right.... Just not related to the O2 sensors at all. (Well, except that they, too, have temperature protections put onto them, just like the catalyst and the exhaust manifold.)

Also, "dump(ing) more fuel into the cylinders to cool the cats down" will have the opposite effect.